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United States Patent |
5,098,549
|
Friese
,   et al.
|
March 24, 1992
|
Sensor element for limiting current sensors for determining the .lambda.
value of gas mixtures
Abstract
A sensor element for limiting current sensors for determining the .lambda.
value of gas mixtures is proposed which has two interconnected inner pump
electrodes (8 and 8') disposed opposite each other in a diffusion channel
(7). The resulting increase of the effective surface at the beginning of
the inner electrodes markedly reduces the disadvantageous electrode
polarization which occurs in sensor elements having only one inner pump
electrode. At the same time, a better utilization of the noble metal
required to form the pump electrodes is achieved.
Inventors:
|
Friese; Karl-Hermann (Leonberg, DE);
Grunwald; Werner (Gerlingen, DE)
|
Assignee:
|
Robert Bosch GmbH (Stuttgart, DE)
|
Appl. No.:
|
490667 |
Filed:
|
February 27, 1990 |
PCT Filed:
|
August 20, 1988
|
PCT NO:
|
PCT/DE88/00512
|
371 Date:
|
February 27, 1990
|
102(e) Date:
|
February 27, 1990
|
PCT PUB.NO.:
|
WO89/02074 |
PCT PUB. Date:
|
March 9, 1989 |
Foreign Application Priority Data
Current U.S. Class: |
204/425; 204/424; 204/426 |
Intern'l Class: |
G01N 027/56 |
Field of Search: |
204/424,425,426
|
References Cited
U.S. Patent Documents
4450065 | May., 1984 | Yamada | 204/412.
|
4505807 | Mar., 1985 | Yamada | 204/425.
|
Foreign Patent Documents |
0142993 | May., 1985 | EP.
| |
0188900 | Jul., 1986 | EP.
| |
0194082 | Sep., 1986 | EP.
| |
3632456 | Apr., 1987 | DE.
| |
0011530 | May., 1980 | FR.
| |
Primary Examiner: Niebling; John
Assistant Examiner: Leader; William T.
Attorney, Agent or Firm: Striker; Michael J.
Claims
We claim:
1. A sensor element formed as a pump cell for limiting current sensor for
determining the .lambda. value of lean exhaust gas of an internal
combustion engine, comprising one of a planar solid electrolyte and a
solid electrolyte in the form of a film, said solid electrolyte conducting
O.sup.2- ions and being formed with a diffusion channel (7) for the gas
under test, an outer pump electrode (9) disposed on the solid electrolyte
(5, 6; 13, 14) and connected to a first conductor track (10), and two
inner pump electrodes (8, 8') arranged on the solid electrolyte in the
diffusion channel (7) opposite the outer pump electrode (9) and on
opposite sides of the diffusion channel (7), the inner pump electrodes (8,
8') being directly connected one to another and to a second conductor
track (10) extending into the diffusion channel (7).
2. Sensor element according to claim 1, characterized in that the inner
pump electrodes (8, 8') are disposed on different ones of said solid
electrolytes (5, 6; 13, 14) assembled by a laminating process.
3. Sensor element according to claim 1, characterized in that between the
first conductor track (10) and the solid electrolyte an insulating layer
(19) is disposed.
4. Sensor element according to claim 2, further comprising a heater (18)
contained between two additional solid electrolytes (15, 16) assembled by
a lamination process.
5. Sensor element according to claim 1, characterized in that the sensor is
manufactured by ceramic-film and screen-printing technology.
6. Sensor element according to claim 1, characterized in that a porous
covering layer is disposed above the outer pump electrode (9) and the
associated first conductor track (10).
7. Sensor element according to claim 1, characterized in that the inner
pump electrodes (8, 8') and the outer pump electrode (9) are annular, and
the solid electrolyte is provided with an inlet hole (17) joining the
diffusion channel (7) for the gas under test, the inlet hole (17) passing
though the inner pump electrodes (8, 8') and the outer pump electrode (9).
8. Sensor element according to claim 1, characterized in that the diffusion
channel 7 contains a porous filling which acts as diffusion barrier.
Description
BACKGROUND OF THE INVENTION
The invention is based on a sensor element for limiting current sensors for
determining the .lambda. value of gas mixtures, particularly of exhaust
gases of internal combustion engines. In such sensor elements, which
employ the principle of the diffusion limiting current, the limiting
current is measured at a constant voltage applied to both electrodes of
the sensor element. In exhaust gas produced in combustion processes, this
current is dependent on the oxygen concentration as long as the diffusion
of the gas to the pump electrode determines the rate of the reaction
occurring. It is known to construct such sensors employing the
polarographic principle of measurement in a manner such that both the
annode and the cathode are exposed to the gas to be measured, the cathode
having a diffusion barrier in order to achieve operation in the region of
the diffusion limiting current.
The known limiting current sensors serve, as a rule, to determine the
.lambda. value of gas mixtures, which denotes the "total oxygen/oxygen
required for complete combustion of the fuel" ratio of an air/fuel mixture
combusting in a cylinder. The sensors determine the oxygen content of the
exhaust gas by a change in electrochemical potential.
Owing to a simplified and cheap method of manufacture, the manufacture of
sensor elements produced by ceramic-film and screen-printing technology
has become established in practice in recent years.
In a simple and efficient manner, planar sensor elements can be produced on
the basis of planar solid electrolytes or oxygen-conducting solid
electrolytes in the form of a film, for example, from stabilized zirconium
dioxide, which are coated on both sides with one inner and outer pump
electrode each, having the associated conductor tracks. At the same time,
the inner pump electrode is advantageously disposed in the peripheral
region of a diffusion channel through which the gas under test is supplied
and which serves as a gas diffusion resistor.
From German Offenlegungsschrift 3,543,759 and also EP-A-O, 142,993, O,
188,900 and O, 194,082, sensor elements and detectors are furthermore
known which have in common the fact that they each have a pump cell and a
sensor cell which comprise planar solid electrolytes or oxygen-conducting
solid electrolytes in the form of a film and two electrodes disposed
thereon and have a common diffusion channel.
A disadvantage of the prior art sensor elements is that the front section,
facing the gas under test, of the inner pump electrode is subjected to a
severer load than the rear section facing away from the gas under test.
This results in a high electrode polarisation which requires a high pump
voltage. The latter in turn entails the danger of an electrolyte
decomposition in the region of the inner pump electrode.
SUMMARY OF THE INVENTION
According to the invention, the sensor element formed as a pump cell for a
limiting current sensor for determining the .lambda. value of lean exhaust
gas of an internal combustion engine, comprises a planar solid electrolyte
or solid electrlyte in the form of a film, which conducts O.sup.2- ions
and is formed with a diffusion channel for the gas under test, an outer
pump electrode disposed on the solid electrolyte and connected to a first
conductor track, and two inner pump electrodes arranged in the diffusion
channel opposite the outer pump electrode and on opposite sides of the
diffusion channel, the inner pump electrodes being directly connected one
to another and to a second conductor track extending into the diffusion
channel.
In contrast to this, the sensor element according to the invention having
an outer pump electrode and two interconnected inner pump electrodes
arranged opposite each other in the diffusion channel, has the advantage
that the effective surface of the inner pump electrode is increased at the
beginning of the electrode and the necessity of increasing the pump
voltage does not arise. The extension of the area of the pump electrode
into the depth of the diffusion channel can remain correspondingly low.
The invention consequently makes possible a better exploitation of the
quantity of noble metal, for example platinum, needed to form the pump
electrodes. A reduced drop in the limiting current furthermore occurs with
increasing electrode ageing compared with the conventional arrangement of
the inner pump electrode on only one side of the diffusion channel.
The sensor element according to the invention can be used instead of known
sensor elements of planar structure in limiting current sensors of
standard design. In this connection, broad-band sensors (.lambda. 1) and
lean sensors (.lambda.>1) are suitable. The sensors according to the
invention may consequently be formed only as a pump cell, possibly with a
heating element, for example as a lean sensor for diesel engines and
incorporated as such in a standard sensor housing, for example of the type
known from German Offenlegungsschrift 3,206,903 and 3,537,051 and used to
measure the fuel/air ratio in a lean or rich exhaust gas. In addition to
the pump cell, the sensor element according to the invention may also
have, in addition, a sensor cell (Nernst cell) which is provided with an
additional air reference channel and one electrode of which is disposed in
the region of the pump electrode in the diffusion channel of the pump cell
and the other electrode of which is situated in the air reference channel.
In another embodiment of the sensor element of the invention, the inner
pump electrodes can be mounted on different solid electrolytes assembled
by a laminating process. Advantageously, an insulating layer is disposed
between the first conductor track and the solid electrolyte on which it is
mounted. This sensor element can also have a heater contained between two
additional solid electrolytes assembled by a lamination process.
A porous covering layer is advantageously provided above the outer pump
electrode and the associated conductor track.
In another embodiment of the invention, the inner pump electrodes and the
outer pump electrode are annular electrodes. An inlet hole is provided in
the solid electrolytes, which joins the diffusion channel and inlet hole
passes through the annular inner pump electrode and the outer pump
electrode.
The sensor element according to the invention is advantageously
manufactured by ceramic-film and screen-printing technology.
The diffusion channel can contain a porous filling, which acts as a
diffusion barrier.
BRIEF DESCRIPTION OF THE DRAWING
The objects, features and advantages of the present invention will now be
illustrated in more detail by the following detailed description,
reference being made to the accompanying drawing in which:
FIG. 1 is a diagrammatic cross-sectional view through a prior art sensor
element including a planar solid electrolyte as carrier and an outer and
inner pump electrode;
FIG. 2 is a diagrammatic cross-sectional view through a sensor element
according to the invention including ceramic films on which the inner and
outer pump electrodes have been printed; and
FIG. 3 is a diagrammatic cross-sectional view through another sensor
element according to the invention, in which the inner pump electrodes and
the outer pump electrodes are annularly disposed around the inlet for the
gas under test.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows the operating principle of a prior art sensor element
comprising a planar solid electrolyte 1 as carrier for the outer pump
electrode 2 and the inner pump electrode 3, which is disposed in the
diffusion channel 4. The pump electrodes 2 and 3 are connected by
schematically illustrated conductor tracks to a source of constant voltage
u, and the current density of O.sup.2- is determined by indicator ions
J.sub.P. The diagrammatic representation of the partial pressure PO.sub.2
of oxygen vs current density of O.sup.2- ions reveals the severer loading
of the inner electrode 3 at the side facing the opening of the diffusion
channel 4, since the oxygen partial pressure drops with the depth of the
electrode and the current density depends on the oxygen partial pressure.
FIG. 2 shows a sensor element according to the invention which can be
manufactured by ceramicfilm and screen-printing technology. It comprises
the solid electrolyte films 5 and 6 on which the inner pump electrodes 8
and 8' and also the outer pump electrode 9 have been printed together with
the associated conductor tracks 10 and which are laminated together by a
standard inter-lamina binder to form the diffusion channel which comprises
a tunnel. Advantageously, a porous filling 12 which serves as diffusion
barrier for the gas under test has been provided, as shown, in the
diffusion channel 7.
FIG. 3 shows another advantageous embodiment of a sensor element according
to the invention, which can be manufactured by ceramic-film and
screen-printing technology and in which the inner pump electrodes 8 and 8'
and also the outer pump electrode 9 are in this case annularly disposed
around the inlet hole 17 for the gas under test. As shown in FIG. 3, the
inlet hole 17 passes through the planar or film solid electrolytes 13, 14,
15, 16 and joins the diffusion channel 7. It is composed essentially of
four solid electrolyte films 13, 14, 15 and 16 which have been laminated
together and which have the punched-out inlet hole 17 of the diffusion
channel 7 for the test gas, the annular outer pump electrode 9 and the two
annular inner pump electrodes 8 and 8' which are disposed opposite
each-other in the diffusion channel 7. The sensor element may furthermore
have a heater 18. The films 15 and 16 with the heater are not, however,
absolutely necessary. The annular electrodes 8, 8' and 9 exposed to the
gas under test are connected to the conductor tracks 10, an insulating
layer 19, for example an Al.sub.2 O.sub.3 layer, being disposed beneath
the conductor track 10. The conductor tracks are connected to a voltage
source which is not shown, for example a battery having a constant working
voltage in the range from 0.5 to 1.0 V. Advantageously, the outer pump
electrode 9 and the associated conductor track 10 are covered by a porous
covering layer 21, for example of magnesium spinel.
Suitable solid electrolytes which conduct oxygen ions for manufacturing
sensor elements according to the invention are, in particular, those based
on ZrO.sub.2,HfO.sub.2, CeO.sub.2 and ThO.sub.2. The use of laminae and
films of yttriumstabilized zirconium dioxide (YSZ) has proved to be
particularly advantageous. At the same time the laminae and films have
preferably a thickness of 0.25 to 0.3 mm.
The pump electrodes are preferably composed of a metal of the platinum
group, in particular platinum, or of alloys of metals of the platinum
group or alloys of metals of the platinum group with other metals.
Advantageously, they contain a ceramic supporting structure material, for
example, YSZ powder, having a proportion by volume of, preferably, about
40% by volume. They are porous and have a thickness of, preferably, 8 to
15 .mu.m. The conductor tracks associated with the pump electrodes are
composed, preferably, also of platinum or a platinum alloy of the type
described. Pump electrodes and conductor tracks can be deposited by known
methods on the solid electrolyte carrier, for example by screen printing.
As a rule, there is an insulation layer, for example of Al.sub.2 O.sub.3,
between the conductor track connecting the outer pump electrode to a
voltage source not shown in the drawing and the solid electrolyte film 13.
It may, for example, have a thickness of about 15 .mu.m. The individual
films or laminae forming the sensor element can be joined together a
method standard in ceramic-film and screen-printing technology in which
the films are assembled and heated to temperatures of about 100.degree. C.
During this process the diffusion channel can be prepared at the same
time. Advantageously, the latter is introduced using thicklayer
technology, for example by a theobromine screen-printed area, the
theobromine being evaporated during the subsequent sintering process. To
produce the diffusion channel, use may also be made, for example, of
thermal soot powders which burn out during the sintering process, or
ammonium carbonate, which evaporates.
If the diffusion channel is to have a porous filling, instead of a
theobromine screen-printed layer, it is possible to use, for example, a
layer of theobromine or another vaporizable or combustible material and a
material that still does not sinter compactly at the sintering temperature
of the solid electrolyte substrate, for example coarse-grain ZrO.sub.2, Mg
spinel or Al.sub.2 O.sub.3 with a grain size of, for example, 10 .mu.m.
EXAMPLE
To produce the sensor element of the type shown diagrammatically in FIG. 3,
films of yttrium-stabilized zirconium dioxide having a layer thickness of
0.3 mm were used. The pump electrodes, which are composed of platinum,
were deposited on the carrier films by known screen-printing technology, a
20 .mu.m thick Al.sub.2 O.sub.3 insulation layer being deposited on the
surface of the carrier films carrying the outer pump electrode in the
region of the conductor track of the outer pump electrode beforehand. The
annular pump electrodes had an outside diameter of 2.8 mm and an inside
diameter of 1.4 mm, with a thickness of 12 .mu.m. The conductor tracks
were produced on the basis of a standard Pt Cermet paste composed of 85
parts by weight of Pt powder and 15 parts by weight of YSZ powder. The
diffusion channel was introduced using thick-layer technology by means of
a theobromine screen-printed layer, the theobromine being evaporated in
the temperature range around 300.degree. C. to leave behind an annular gap
about 30 .mu.m high and 1.3 mm deep. The central inlet opening for the gas
under test had a diameter of 0.25 mm. After printing the carrier films,
ie. after depositing the electrodes, conductor tracks, insulating layer
and also, possibly, a covering layer on the outer pump electrode, the
films were subjected after assembly to a sintering process in which they
were heated for about 3 hours at a temperature in the region of
1380.degree. C.
To manufacture a further sensor element with a heater, as shown
diagrammatically in FIG. 3, further films 15,16 with a printed-on heater
18 were laminated on before heating.
The sensor elements manufactured were incorporated in the sensor housing of
the type known from the German Offenlegungsschrift 3,206,903 and 3,537,051
and used to measure the fuel/air ratio in Lean and rich exhaust gases.
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